Mine appear light grey in preview. The preview render is a quick/crude portrayal. If it renders properly, then it's OK.

If it doesn't render properly, there may be improper node connections in the glass material. If this little icon
(which looks like a cartoon speech balloon) turns from grey to yellow when you render, click on it to open the message log; it will specify any disallowed node connections.

Another reason for black glass in Superfly renders is insufficient bounces.
There is an overall limit (max bounces) and individual limits for particular effects (transmission, for refraction) and transparency, volume, etc.

Set these limits high and Superfly will use as many bounces as needed to complete a raytrace.